On the excited states ofp-quinones and an interpretation of the photocycloaddition ofp-quinones to alkenes

Abstract

A theoretical investigation is made of the electronic states ofp-benzoquinone (PBQ), methyl substituted PBQ's and 1,4-naphthoquinone (NQ). In accord with experiment, the lowest triplet state of PBQ is calculated to be3B1g (n, π*), while that for duroquinone (DQ) is3B3g (π, π*). The electron densities of these states are consistent with the hypothesis that3n, π* states lead to oxetan formation and3π, π* states to cyclobutanes. It is predicted that trimethyl PBQ might form both adducts, as the two states are calculated to be nearly degenerate.

The photochemistry of NQ is more complex. The lowest excited triplet state is calculated to be ofn, π* symmetry, in accord with experiment; however, several other states are predicted near in energy, and the photochemistry cannot be rationalized unambiguously.